This invention relates to an optical encoder which is applicable to positional measurement for machine tools such as lathes, milling machines or the like or semiconductor manufacturing systems.
FIG. 1 is a perspective structural view to showing an example of an optical system in a prior art absolute type optical encoder which comprises a luminous element 11 such as a LED (light emitting diode), a lamp or the like which emits light La for measurement, a collimating lens 12 which collimates the light La emitted from the luminous element 11 into parallel light Lb, a first scale 13 which has n (n is an integer) grating tracks t.sub.1, t.sub.2, . . . , t.sub.n arranged on the surface in parallel, each grating track comprising sections 13A (referred to as transmission sections herein) which allow the parallel light Lb which has passed through the collimating lens 12 to pass and sections 13B (referred to as non-transmission sections herein) which does not allow the parallel light Lb to transmit therethrough alternating in a predetermined length (referred to as a grating pitch herein), a second scale 14 which is provided with transmission windows 14A.sub.1, 14A.sub.2, . . . , 14A.sub.n which allow light beams (not shown) passing through the transmission sections 13A to pass and are arranged in correspondence to the respective grating tracks t.sub.1, t.sub.2, . . . , t.sub.n of the first scale 13, and photo detectors 15-1, 15-2, . . . , 15-n which are arranged in correspondence to the respective transmission windows 14A.sub.1, 14A.sub.2, . . . , 14A.sub.n so as to convert light beams L.sub.c1, L.sub.c2 . . . , L.sub.cn passed through the respective transmission windows 14A.sub.1, 14A.sub.2, . . . , 14A.sub.n into electric signals according to their intensities.
The first scale 13 used in the optical system 10 in the prior art absolute type optical encoder is provided with reflected binary codes (Gray codes) as shown in FIG. 2 wherein grating pitches P.sub.1, P.sub.2, P.sub.3, . . . , P.sub.n-1, P.sub.n between the adjacent grating tracks t.sub.1 and t.sub.2 ; t.sub.2 and t.sub.3 ; . . . ; t.sub.n-1 and t.sub.n are in a ratio of 1:2. Accordingly, the intensities of the light beams L.sub.c1, L.sub.c2, L.sub.c3, . . . , L.sub.cn-1, L.sub.cn which have transmitted through the transmission sections 13A of the respective grating tracks t.sub.1, t.sub.2, t.sub.3, . . . , t.sub.n-1, t.sub.n of the first scale 13 and the transmission windows 14A.sub.1, 14A.sub.2, 14A.sub.3, . . . , 14A.sub.n-1, 14A.sub.n of the second scale 14 corresponding to the grating tracks t.sub.1, t.sub.2, t.sub. 3, . . . , t.sub.n-1, t.sub.n of the first scale 13 and entered the respective photo detectors 15-1, 15-2, 15-3, . . . , 15-n-1, 15-n corresponding to the transmission windows 14A.sub.1, 14A.sub.2, 14A.sub.3, . . . , 14A.sub.n-1, 14A.sub.n periodically change since the first scale 13 moves in a longitudinal direction (the direction marked by an arrow m). Responsive to the changes, the electric signals converted by the respective photo detectors 15-1, 15-2, 15-3, . . . , 15-n-1, 15-n change periodically. FIG. 3 shows such periodical changes of the electric signals S.sub.1, S.sub.2, S.sub.3, . . . , S.sub.n-1, S.sub.n in a graph wherein a displacement ml of the first scale 13 in the longitudinal direction is plotted on a horizontal axis while the electric signal S.sub.1, S.sub.2, S.sub.3, . . . , S.sub.n-1, S.sub.n converted by the respective photo detectors 15-1, 15-2, 15-3, . . . , 15-n-1, 15-n are plotted on a vertical axis. FIG. 4 is a block diagram of an absolute type optical encoder wherein the electric signals S.sub.1, S.sub.2, S.sub.3, . . . , S.sub.n-1, S.sub.n are digitized by respective comparators 20 into digital signals d.sub.1, d.sub.2, d.sub.3, . . . , d.sub.n-1, d.sub.n and further converted from the reflected binary codes into an absolute positional data D of desired form such as simple binary codes or a BCD code by a decoder 30.
There are demands to improve a resolutional detection of a conventional absolute type optical encoder so as to enable detection of minute displacements, and to enable detection of absolute positions of a longer stroke. However, the conventional absolute type optical encoder is defective in that its resolutional detection is limited in such a way that the detection of smaller positions than the grating pitch P.sub.n in the track t.sub.n, which is segmented into the smallest possible segments, is impossible, and the detection stroke in its absolute positional detection remains at a level similar to the grating pitch P.sub.1 of the track t.sub.1 which is segmented into the largest possible segments. If an attempt is made to improve its resolutional detection and to extend the detection stroke, the number of grating tracks inevitably increases to enlarge the size of the absolute type optical encoder, or to inconveniently increase the number of compounds such as photo detectors and comparators.